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umask() sets the calling process's file mode creation mask (umask) to
mask & 0777 (i.e., only the file permission bits of mask are
used), and returns the previous value of the mask.

The umask is used by open(2), mkdir(2), and other system calls
that create files to modify the permissions placed on newly created files or
directories. Specifically, permissions in the umask are turned off from the
mode argument to open(2) and mkdir(2).

Alternatively, if the parent directory has a default ACL (see acl(5)),
the umask is ignored, the default ACL is inherited, the permission bits are
set based on the inherited ACL, and permission bits absent in the mode
argument are turned off. For example, the following default ACL is equivalent
to a umask of 022:

u::rwx,g::r-x,o::r-x

Combining the effect of this default ACL with a mode argument of 0666
(rw-rw-rw-), the resulting file permissions would be 0644 (rw-r--r--).

The constants that should be used to specify mask are described in
inode(7).

The typical default value for the process umask is
S_IWGRP | S_IWOTH (octal 022). In the usual case where
the mode argument to open(2) is specified as:

S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH

(octal 0666) when creating a new file, the permissions on the resulting file
will be:

A child process created via fork(2) inherits its parent's umask. The
umask is left unchanged by execve(2).

It is impossible to use umask() to fetch a process's umask without at the
same time changing it. A second call to umask() would then be needed to
restore the umask. The nonatomicity of these two steps provides the potential
for races in multithreaded programs.

Since Linux 4.7, the umask of any process can be viewed via the Umask
field of /proc/[pid]/status. Inspecting this field in
/proc/self/status allows a process to retrieve its umask without at the
same time changing it.

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